Process for the production of very low carbon-containing cold-rolled steel strips



United States Patent PROCESS FOR THE PRODUCTION OF VERY LOW CARBON-CONTAINING COLD-ROLLED STEEL STRIPS Nobuo Fukuda, Mineo Shimizu, Kameo Matsukura, and Narumi Ando, Kitakyushu, Japan, assignors to Yawata Iron & Steel Co., Ltd., Tokyo, Japan No Drawing. Filed Aug. 20, 1965, Ser. No. 481,399

US. Cl. 148-12.1 4 Claims Int. Cl. C21d 7/14 ABSTRACT OF THE DISCLOSURE The present invention is concerned with a process for producing a very low carbon cold-rolled steel strip comprising the first step of hot-rolling a low carbon steel containing 0.03 to 0.12 wt. percent carbon, the second step of allowing to cool the thus hot-rolled steel strip to produce a secondary mill scale layer on the surface of said steel strip and thereafter annealing the resultant steel strip in a heating zone containing a non-oxidizing gas to reduce the carbon in the steel to an amount less than 0.010 wt. percent by oxidation with the secondary mill scale on the surface, and the third step of cold-rolling the very low carbon steel strip after decarburization annealing at a reduction rate of 30 to 80%, annealing the cold-rolled strip for recrystallization in a non-oxidizing gas and thereafter subjecting the annealed steel strip to a temper-rolling.

This invention relates to a process for the production of a very low carbon-containing cold-rolled steel strip having an extremely high enameling property or a onestep enameling property.

It is an object of this invention to produce a very low carbon cold-rolled steel sheet or strip having a good formability and a particularly excellent enameling property, for example, an enameling low carbon steel sheet suitable for bathtub materials and the like.

Another object of this invention is to provide an economical process for the production of an extremely low carbon steel strip for high-grade enameling having a high enameling property and an extremely low sagging without using any specific atmospheric gas in the decarburization annealing, or using an inexpensive non-oxidizing atmospheric gas usually used for annealing thin steel strip.

As one process for producing enameling steel sheet, there is known a process wherein the content of carbon in the steel'is reduced to about 0.030% and at the same time the contents of manganese, phosphorus and the like which are injurious to the enameling property of steel are reduced in the steel manufacturing step.

The steel sheet produced by the conventional process has generally an excellent enameling property but the enameling procedure for the steel cannot be performed by one coat enameling and also the drawability of the steel sheet is not good. Further, there are such drawbacks in the above mentioned known process that skimming must be conducted in the steel-manufacturing step and that the danger of causing hot shortness in the hot-rolling step owing to the presence of sulfur in the steel is very great even though a specific rolling process is adopted.

A conventional process has also been provided for producing enameling steel sheets by fixing carbon in the steel by titanium but such a process has been used in only a limited case since there are many problems in the process and in the qualities of the products. For example, the addition of titanium becomes very difficult as the carbon content in the steel is reduced, the yield is greatly lowered due to the formation of killed steel, the surface property of the steel sheet is reduced.

3,420,718 Patented Jan. '7, 1969 "ice It may be considered that an enameling low carbon steel sheet is produced by applying a process known as a decarburization process for steel, that is, by applying a vacuum degassing process wherein a molten steel from a steel-manufacturing step is subjected to vacuum degassing, or an open coil decarburization annealing process wherein a steel strip coil is in a state of loose coil, subjected to decarburization annealing in a wet hydrogen atmosphere, or a process wherein the carbon in steel is oxidized and removed by treating the surface of a cold-rolled steel strip coated with iron oxide powders. However, there are many troubles in such processes also.

That is, there is a drawback in the application of the vacuum degassing process, although the steel decarburized by the vacuum degassing process is excellent in cleanness of steel itself; that the application of the vacuum degassing process needs complicated and specific apparatus and gives a low productive power, renders the application of the process for the production of a usual low-carbon steel too expensive.

Further, an enameling iron sheet obtained by the application of the open coil type decarburization annealing process, that is, by decarburizing rimmed cold-rolled steel strip in a state of loose coil in an atmosphere containing wet hydrogen to extremely reduce the carbon content and temper-rolling it in a tight coil state is excellent in sagging as compared with usual coil-rolled steel strip as well as with an enameling pure-iron sheet, and at the same time can be enameled in one step 'and has good formability, but it is very difficult by such process to produce an enameling steel strip of thickness above about 1.6 mm. The reasons are as follows; that is, in the open coil decarburization annealing method, since a cold-rolled steel strip is subjected to decarburization annealing in a loose coil state, the steel strip coil must be in a tight coil state again prior to applying temper-rolling after decarburization annealing but since the diameter of a roll for temper-rolling is about 20 inches, the thickness of steel strip that is not broken when the strip is rolled around the roll having such a diameter must be less than about 1.6 mm., and in order to carburization anneal a cold-rolled steel strip having thickness above 1.6 mm. in a conventional open coil type decarburization annealing equipment, the annealing must be conducted by increasing the diameter of the rolled steel coil as large as possible, in other words, re-. ducing the number of coil-rolling as small as possible, which makes the application of the open coil type decarburization annealing unprofitable in economy.

Furthermore, in a process for removing carbon in steel by applying iron oxide powders on the surface of a cold-rolled steel strip and heating in an inert gas to oxidize the carbon, there are troubles that uniform coating of the oxidizing agent is usually difiicult and the process requires a coating means, a cleaning means for the surface of steel strip after decarburization and complicated procedures therefor.

In accordance with the process of this invention, a very low carbon-containing cold-rolled steel sheet, in particular an enameling thick steelsheet, can be effectively produced without being accompanied by the above-mentioned coil breaking phenomenon and without need of complicated apparatus and procedures. That is, in the process of this invention, the decanburization annealing is conducted not in a wet hydrogen atmosphere as in an open coil annealing method but by covering the surface of a steel strip with surface scales formed by cooling the strip after hot-rolling, therefore, rolling operators, without having an open coil annealing installation, can produce comparatively easily a steel strip having particularly excellent enameling property and formability. The installation for open coil annealing apparatus required a large investment for equipment but since :by the process of this invention an enameling thick steel sheet that can not be economically produced by the open coil annealing process can be produced without need of such an economical burden, the economical effect of the process of this invention is great.

By the process of this invention the necessary decarburization annealing is applied to a hot-rolled steel strip in a state covered by scales prior to conducting a cold-rolling, that is, after hot-rolling. The cold-rolling in this invention is conducted after decarburization, with a reduction ratio of 30-80%.

After cold-rolling, the steel strip is subjected to recrystallization annealing and then to a temper-rolling with a reduction ratio of 0.5-3%. The amount of the scales on the surface of the steel strip formed after hot-rolling is not strongly affected by the hot-rolling temperature and rolling conditions, and by cooling the hot-rolled coil after continuous hot-rolling in the air, the scales are uniformly formed.

At that, since the surface of the steel strip has a scale forming faculty until the steel strip is cooled to below 300 C., the temperature decreasing velocity and humidity until the steel strip is cooled from the hot-rolling temperature to 300 C. must be properly checked to form a necessary amount of scale for subsequent decarburization annealing.

The process of this invention has an effect that the decarburization ennealing can be carried out without adopting an open coil type decarburization annealing method in a wet hydrogen atmosphere, that is, the decarburization and annealing for steel can be effected at the same time in the state of tight coil and using a conventionally used Hz-Nz non-oxidizing atmosphere. This is caused "by annealing the steel strip coil in a state having secondary mill scales on the surface after hot-rolling. Although there may be a trouble of recoiling, by applying an open coil type decarburization annealing process to a hot-rolled steel strip, uniform and rapid decarburization can of course be effected.

The process for the production of very low carboncontaining cold-rolled steel strip by this invention is explained in detail as follows;

The steel is first produced in an openhearth furnace or a converter, and the removal of unnecessary components and adjustment of other components are conducted by usual known means according to the uses of thus produced steel. Further, in order to carry out the decarburization effectively in the subsequent annealing procedure, the carbon content in steel must be reduced to 0.12-0.030 percent by weight. Thus produced molten steel is cast into an ingot and although the kind of steel may be rimmed steel or semi-killed steel, rimmed steel is most general when produced for enameling steel sheet.

Then, the steel ingot is slabbed under uniform heating into steel slabs, which are, after being subjected to a necessary surface treatment, heated and hot-rolled. The holt-rolling finishing temperature is 8709l0 C., the steel strip is coiled at about 600 C., and allowed to cool in the air to room temperature.

In cooling in the air, an iron oxide layer or secondary mill scale is formed on the surface of the hot-rolled steel strip and if a sufficient amount of the iron oxide layer for decarburization is formed, it is unnecessary to cool the plate further, but for preventing the danger of treating the hot steel strip coil, it is generally desirable to cool it to room temperature.

The surface scale of the hot-rolled steel coil is influenced in the thickness of the formed scale and the scale layer by the oxidation condition until the temperature of the steel strip is reduced to 300 C. and by gradual cooling from a usual hot-rolling finishing temperature, three layers of a wustite layer (FeO), a magnetite layer (Fe O and an a-hematite layer (Fe O' are formed. When the scale is thickened, cracks are formed in the magnetite layer and a-hematite layer is formed along the cracks. Further, the small particles of iron and magnetite are intermingled in the wustite layer in the cooling step, which contributes to carrying out decarburization annealing effectively. The scale is usually formed in the thickness of about 10 microns in one side.

Thereafter, the hot-rolled steel strip having the iron oxide layer or the secondary scales necessary for decarburization annealing is subjected to decarburization annealing in a conventional annealing furnace for thin plates.

As the annealing gas, there may be used an H N inert gas usually used for annealing thin steel strip coil, such as an HNX gas, an NX gas, and an AX gas and N gas as a waste gas from an oxygen production by an air liquefaction process.

In the process of this invention, since the hot-rolled steel strip is annealed in a state having the secondary scales on the surface to reduce the carbon content in the steel strip by the scales attached on the surface of the steel strip, the decarburization can be carried out effectively without using a specific gas atmosphere as in an open coil decarburization annealing process. The steel strip coil is annealed using an N gas or an Hg-Nz inert gas (e.g., an HNX gas) and maintained at the annealing temperature until the content of CO in the waste gas becomes less than 1.0%. In order to conduct the decarburization effectively, the most suitable annealing temperature is between the A transformation point and 650 C. An annealing temperature outside the temperature range is not preferable since the decarburization rate is reduced. Further, a forcible decarburization may also be carried out by adding wet hydrogen in the above-mentioned annealing atmosphere but the main object of this invention is not concerned with the presence of such wet hydrogen in the annealing atmosphere. However, by applying such a forcible decarburization, the annealing period of time can be reduced.

By the annealing treatment, the carbon in steel is oxi dized by the oxygen of the scales on the surface of the steel strip and removed therefrom down to 0.01-0.0005 percent by weight, which gives good deep drawing.

Thus obtained decarburized low carbon steel strip is pickled and cold-rolled into desired dimensions by a known process. In this case, there is no limitation in general in the reduction ratio but the strip is cold-rolled with a usual reduction ratio, that is, the reduction ratio of 3080% to the final thickness.

In order to-further improve the deep drawa bility of the steel sheet, it is desirable to carry out the cold-rolling with the reduction ratio of 6070%. The low carbon steel strip cold-rolled into the final thickness is further subjected to bright recrystallization annealing in an inert gas atmosphere and then to temper-rolling. I

The conditions, such as atmosphere and temperature, for the aboveamentioned bright recrystallization annealing maybe known ones, for example the annealing may be carried out in an H N inert gas such as an HNX gas and at temperatures between 600 C. and the A transformatlon point. The temper-rolling may be conducted with a reduction ratio of below 3% preferably 0.5-1.5

One of the important properties for excellent enameling low carbon steel sheets is that the strain of the steel sheet itself at high-temperature baking, that is, sagging is small and among various steel plate products produced by various processes in same conditions, sagging of the product by the process of this invention and the enameling low carbon steel sheet decarburized by an open coil annealing process is smallest. Moreover, while one-step enameling cannot be adopted in a cold-rolled rimmed steel sheet and pure-iron sheet due to the formation of defects, it is possible in the low carbon steel sheet obtained by the present invention and adhesive property in this case is better. Further, the mechanical properties of the steel sheet by this invention, such as the yield point, the total elongation 6 and the Erichsen value are almost the same as those of Thus obtained low carbon steel sheet was investigated a cold-rolled rimmed steel sheet and the former has a with respect to sagging, appearance by one-step enamelfor'mability the same as that of the latter. ing, adhesive property of enameling, and mechanical The carbon content in the product according to this properties, which showed they were almost same as those invention is about 0.002 percent by weight, or about the of the low carbon steel sheet obtained by an open coil same as that of an enameling low crabon steel sheet 5 type decarburization annealing of acold-rolled strip. decarburized by the open coil annealing process and other components are in ranges of usual low carbon steel. EXAMPLE 2 A5 mentionfid above, Since in the Process for the A hot-rolled steel strip coil of 5.7 mm. in thickness was duction of low carbon steel strip, the decarburization 10 prepared under the same conditions as in Example 1 and annhahng is conducted by utilizing iron oxide finely cooled to form finally the desired secondary scale on the formed on the surface of a hot-rolled steel strip, it is not Surface f the hohrohed strip The hotq-ohed Strip was necessary to apply Specific iron oxide for decarburization then in a tight coil stale subjected to decarburization anon the surface of a steel strip to be treated and to remove healing in an inert gas having a dew point f 40 the residues on the surface after decarburization, which and consisting of 5% H2, the remainder being N2, at makes the process of this invention very efiicient. Morec for the total annealing period of 37 hours and over, in the process of this invention, there are no restricthe Soaking period of 25 home The components in the tions in Procedure for hot-Toning i the h of the decarburized low carbon steel strip were almost the same above'mehhohed Process for Prodhclhg Purehoh Sheet as in Example 1 and a slight deviation was observed in or htahihm'hxed ehhmehhg steel Sheet as Well as Yleld the carbon content in the steel but the properties of the surface Properties of the Steel Sheets by the mveh' steel strip were not influenced by such a small deviation tion are excellent since a rimimed steel is used in this of carbon content invention By applying to the steel strip coil recrystallization an- Fhrthenhore, slhce the Stee1 StnP h f nealing and temper-rolling with the reduction ratios the after the decarburization annealing in this invention, the Same as in Ex'ample 1, the tempered rolled Steel Strip of defects such as edge breaking at decarburization anneal- 20 mm. in thickness was Obtained ing and the like are completehf rfimovefi h Then the tempered rolled steel strip was, according to Therefore by the Process of thls lhvehtloh ehamehhg the standard of the Porcelain Enamel Institute in U.S.A., low carbon steel sheet having the thickness of about 1.6 ground coated in a test piece of X and heated for to 2.5 mm. and over that has never been obtained by con- 10 minutes at with the supporting distance of vehtlohal Processes can be produced' 10', whereby the change of strains before and after heat Exemplary embodiments of this invention are shown as f ll i g Was defined as sagging, the results being compared 0 ows:

with saggings of a rimmed cold-rolled steel sheet, an EXAMPLE 1 enameling pure-iron sheet, and an open coil annealed A molten steel containing 0.08% C, 0.01% Si, 0.017% enameling steel sheet. Further, according to the standard S, 0.017% P, and 0.35% Mn was cast into an ingot, which of the Porcelain Enamel Institute, the unstripped area in was, after sl-abbing, hot-rolled at a finishing temperature the case of giving a brattice distortion of a definite depth of 880 C. into a steel strip coil of 5.7 mm, in thickness. was measured, that is, adherence index or adhesiveness The strip was coiled at 650 C. and was cooled to room was measured. temperature to form desired fine scales (0.1-0.02 mm. in 40 Moreover, in order to know the mechanical properties, thickness) on the surface of the hot-rolled strip and in a the yield points, total elongation and Erichsen value were loose coil state was subjected to decarburization annealmeasured, the results being shown in the following table ing in an inert gas of 5% H the remainder being N and together with the chemical composition of the products.

TABLE Sagging Adhesive- One-coat Yield Total Erichsen Chemical composition of product Sort (m./m.) ness enameling point elongation value (percent) (kg/mm?) (percent) (In/m.) 0 Mn P S Enameling steel strip in 4 97 Good 16. 5 49 12. 0 0. 002 0. 33 0. 017 0. 017

Example 2. Open-coil decarburization an- 4 97 do. 17 49. 5 12.0 0. 002 0. 32 0. 017 0.017

nealed enameling steel strip. Enameling pure-iron strip 9. 5 Defectl8. 5 46. 5 11. 5 0. 020 0. 02 O. 004 0. 015 Rimmed coldrolled steel strip 15 do 20. 5 49 12. 1 0.080 0.33 0.017 0. 017

1 Measurement impossible.

having a dew point of --40 C. at 700 C. for a total As clear from the above table, in the case of conductannealing period of 30 hours and the soaking period of ing decarburization annealing in a usual non-oxidizing 20 hours. The composition of thus decarburized hot-rolled atmosphere in a tight coil state after hot-rolling, the low strip was as follows, which shows the carbon content becarbon steel sheet made by this invention was almost the ing remarkably reduced. same as the open coil annealed enameling steel sheet in P r n sagging, adhesive property, enameling property and the C 0-002 like. The open coil annealed enameling steel sheet for gin comparative test had 1.6 mm. in thickness.

1 P 0.017 EXAMPLE 3 S 0017 A hot-rolled steel strip having 5 .7 mm. in thickness and The decarburized hot-rolled strip coil was cooled to having the fine secondary mill scales on the surface was room temperature, passed continuously through a pickling prepared under the conditions same as in Example 1. line to clean the surface with an aqueous solution of sul- After being recoiled into a loose coil, it was subjected to furic acid containing some hydrochloric acid, and then decarburization annealing in a wet hydrogen containing cold-rolled with a reduction ratio of 65% to a final digas atmosphere mainly consisting of nitrogen and hydromension. By annealing the cold-rolled strip coil for regen and having a dew point of 5-50" C. at 700 C. for moving strains for 2 hours at 650 C. in an inert gas havthe total annealing period of 25 hours' and the soaking ing a dew point of 40 C. and consisting of 5% H the period of 15 hours. By cold-rolling recrystallizing annealremainder being N and then temper-rolling it with a reing and temper-rolling it with the reduction ratios as in duction ratio of 1%, the tempered and cold-rolled steel strip of 2.0 mm. in thickness was obtained.

Example 1, the tempered steel strip of 2.0 mm. in thickness was obtained.

Sagging, one-step enameling property, and other mechanical properties of the steel plate were almost the same as in Example 2.

What is claimed is:

1. A process for producing a very low carbon coldrolled steel strip comprising the first step of hot-rolling a low carbon steel containing 0.03 to 0.12 wt. percent carbon, the second step of allowing to cool the thus hotrolled steel strip to produce a secondary mill scale layer on the surface of said steel strip and thereafter annealing the resultant steel strip in a heating zone containing a non-oxidizing gas to reduce the carbon in the steel to an amount less than 0.010 wt. percent by oxidation with the secondary mill scale on the surface, and the third step of cold-rolling the very low carbon steel strip after decarburization annealing at a reduction rate of 30 to 80%, annealing the cold-rolled strip for recrystallization in a non-oxidizing gas and thereafter subjecting the annealed steel strip to a temper-rolling.

2. A process for producing a very low carbon steel strip comprising the first step of hot-rolling a low carbon rimmed steel containing 0.03 to 0.12 wt. percent carbon, the second step of allowing to cool the thus hot-rolled low carbon rimmed steel strip to room temperature in the air to produce a fine secondary mill scale layer on the surface of said steel strip and then annealing the resultant steel strip, as it is in the state of a tight coil, in a heating zone containing a non-oxidizing gas to reduce the carbon in the steel to an amount less than 0.010 wt. percent by the oxidation with the secondary mill scale on the surface, and the third step of cold-rolling the very low carbon steel strip after decarburization annealing at a reduction rate of 30 to 80%, annealing the cold-rolled steel strip for recrystallization in a non-oxidizing gas and thereafter subjecting the annealed steel strip to a temper-rolling at a reduction rate of 0.5 to 1.5%.

3. A process for producing a very low carbon coldrolled steel strip comprising the first step of hot-rolling a low carbon rimmed steel containing 0.03 to 0.12 wt. percent carbon, the second step of allowing to cool the thus hot-rolled low carbon rimmed steel strip to room temperature in the air to produce a fine secondary mill scale layer on the surface of said steel strip, and then annealing the resultant steel strip, as it is in the state of a tight coil, in a heating zone containing a non-oxidizing gas at a temperature of from the A transformation point to 650 C., until the CO content in the waste gas becomes less than 1.0%, to reduce the carbon in the steel to an amount less than 0.010 wt. percent by the oxidation with the secondary mill scale on the surface, and the third step of pickling the very low carbon hot-rolled strip after decarburization annealing, thereupon cold-rolling the pickled steel strip at a reduction rate of 30 to 80%, annealing the cold-rolled strip for recrystallization in a non-oxidizing gas at a temperature of from the A; transformation point to 650 C. and thereafter subjecting the annealed steel strip to a temper-rolling at a reduction rate of 0.5 to 1.5%.

4. A process for producing a. very low carbon coldrolled steel strip comprising the first step of hot-rolling a low carbon rimmed steel containing 0.03 to 0.12 wt. percent carbon, the second step of allowing to cool the thus hot-rolled low carbon rimmed steel strip to room temperature in the air to produce a fine secondary mill scale layer on the surface of said steel strip and then annealing the resultant steel strip, as it is in the state of a tight coil, in a heating zone containing a non-oxidizing gas, that is, HNX gas, NX gas, or N gas as a waste gas from an oxygen production by an air liquefaction process, or mixed gas of above-mentioned waste N gas and AX gas, at a temperature of from the A transformation point to 650 C., until the CO content in the waste gas becomes less than 1.0% to reduce the carbon in the steel to an amount less than 0.010 wt. percent by the oxidation with the secondary mill scale on the surface, and the third step of pickling the very low carbon hot-rolled strip after decarburization annealing, thereupon cold-rolling the pickled steel strip at a reduction rate of to annealing the cold-rolled strip for recrystallization in a nonoxidizing gas at a temperature of from the A transformation point to 650 C. and thereafter subjecting the annealed steel strip to a temper-rolling at a reduction rate of 0.5 to 1.5%

References Cited UNITED STATES PATENTS 2,236,519 4/1941 Carpenter l48l2.l 2,602,758 7/1952 Olt et a1. 117l29 3,244,565 4/1966 Mayer et a1 l48l2.l

HYLAND BIZOT, Primary Examiner.

WAYLAND W. STALLARD, Assistant Examiner. 

